This application claims priority, under 35 USC 119, of earlier-filed Japanese application HEI3-26795, filed Jan. 28, 1991.
The present invention relates to a hydraulic (or hydrostatic) power steering device for a vehicle, and more particularly, to such a device for use on vehicles having multiple steerable axles.
In many off-highway vehicles, for example, various tractors and construction vehicles, it is desirable to be able to steer both the front and rear axles. More specifically, it is desirable to be able to steer the front axle in response to rotation of the vehicle steering wheel, and to control the steering of the rear axle by means of hydraulic cylinders and a multi-position control valve. As a result, the rear wheels may be turned in the same direction as the front wheels to provide "crab" steering, or the rear wheels may be turned in a direction opposite that of the front wheels, thus providing a sharper turning radius.
A typical prior art system of the type described above is illustrated in FIG. 3 of the present application, and is illustrated as including a conventional, well-known hydraulic steering unit for steering both the front and rear axles (wheels) of the vehicle. In the system shown in FIG. 3, a gear 50 is fitted on a column 2 connected to a steering wheel 1. A front wheel steering unit (not shown) is connected to the gear 50 through a connecting linkage or member 4. Another gear 51 meshes with the gear 50, and a shaft 52 of the gear 51 is connected to the input shaft of a known hydraulic steering unit 53. The hydraulic steering unit may be of the type illustrated and described in U.S. Pat. No. 4,804,016, assigned to the assignee of the present invention and incorporated herein by reference. A pair of control conduits 55 and 56 are connected to a pair of control ports L and R, respectively, provided in a housing 54 of the hydraulic steering unit 53. The conduits 55 and 56 are connected to an arrangement 58 of steering cylinders for steering the rear wheels through a changeover valve 57, so that the required pressurized fluid may be communicated to the cylinders 58. The reference numeral 59 indicates a fluid supply conduit, connected to an inlet port P of the steering unit, and the reference numeral 60 indicates a fluid discharge conduit connected to the outlet port T of the hydraulic steering unit 53. The fluid supply conduit 59 receives pressurized fluid from a pump 61 while the fluid discharge conduit 60 returns fluid to a system reservoir 63.
Among the types of front wheel steering units which are well known, and therefore not illustrated or described herein, are those of the integral gear or semi-integral gear type, as well as those of the linkage type.
Among the problems associated with the prior art steering system shown in FIG. 3, and just described, is that steering torque input to the front wheel steering unit, and steering torque for input to the hydraulic steering unit 53 are additive, i.e., the total torque input required is merely the sum of the two individual torques.
Another problem with such systems is that the arrangement of gears 50 and 51 would subject the input shaft of the steering unit 53 to a substantial sideload, which is not desirable in such steering units. Therefore, the shaft 52 must be supported by a bearing arrangement capable of preventing the radial loads acting on the shaft 52 from causing any radial deflection thereof. Such a bearing arrangement typically adds substantial expense to the system, and takes up a substantial amount of space.